Ian Tattersall American Museum of Natural History, N ew Yo r k Patterns of origin and extinction in the mammal fauna of Madag a s c a r Tattersall, I., 1999 - Patterns of origin and extinction in the mammal fauna of Madagascar - in: Reumer, J . W. F. & De Vos, J. (eds.) - EL E P H A N T S H AV E A S N O R K E L! PA P E R S I N H O N O U R O F PA U L Y. SO N D A A R - D E I N S E A 7: 303-311 [ISSN 0923-9308]. Published 10 December 1999 M a d a g a s c a r ’s huge size and long history of isolation have assured ongoing debate about both the ori- gin of its oddly constituted fauna and the nature of the major extinction event that occurred following the first incursion of humans onto the island some two thousand years ago. In this paper I briefly review the geological and biological evidence for the origin of the modern Malagasy fauna, and examine the potential roles of direct human intervention and climatic change in the extinction of Madagascar’s l a rge-bodied mammals. Despite the remarkable dearth of evidence for direct interaction between humans and the island’s ‘subfossil’ fauna, available evidence of climatic conditions in Madagascar since the end of the Pleistocene does not indicate that its fauna was under unprecedented environmen- tal stress in this period. It is thus impossible to avoid the conclusion that human activity played a criti- cal role in the elimination of several dozen species of mammals and birds of larger body size than their surviving relatives. In turn, this observation underscores the fact that the documented extinctions do not constitute a static and completed historical phenomenon, but rather form part of an ongoing process that continues today to menace Madagascar’s unique fauna and flora. Correspondence: Ian Tattersall, Department of A n t h r o p o l o g y, American Museum of Natural History, New York, NY 10024, USA. e-mail: [email protected] Keywords: Madagascar, islands, faunal origins, extinctions, mammals I N T RO D U C T I O N A great landmass some 1600 km long and both faunal and floral; but then, so also do lying about 400 km off the southeastern continents, in proportion to their own histo- African coast, Madagascar is routinely refer- ries of isolation. Similarly, apparent cases of red to as ‘the world’s fourth largest island’, giantism (e.g. in the elephant bird, A e p y o r n i s and ‘the world’s largest oceanic island’. m a x i m u s , and in the huge indriid primate Statements such as these place as much emp- A rchaeoindris fontoynonti), and of dwarfism hasis on Madagascar’s sheer size as on its (e.g. in the smallest primate, M i c ro c e b u s insular nature, and point to the fact that this m y o x i n u s , or in the up to three pygmy is a very special kind of island: one that is hippotamus species) can indeed be found in not only impervious to many of the assump- Madagascar; but such examples are better tions of island biogeography, but that in many viewed simply as extremes of the local size ways shows the biotic characteristics of a spectra of the major taxa involved, or explai- continent. It is possible, of course, to find ned (in the case of the pygmy hippos) by instances in Madagascar of many phenomena quite recent immigration. that are generally considered to be characte- ristic of islands; but in most cases these must Nonetheless, one may fairly point to two be hedged with caveats. Thus, for example, major aspects of Madagascar’s biological his- Madagascar shows high rates of endemism, tory that have been critically influenced by its 303 ELEPHANTS HAVE A SNORKEL! DEINSEA 7, 1999 insular nature. The first of these is the origin cal reconstruction speculative at best; but of the Malagasy biota, or at least of its post- relationships among the living African and Gondwanan elements. Here a strong initial Malagasy primates may suggest that at least filtering effect is discernible, together with two successful primate invasions of dramatic post-filter diversification. Thus, for Madagascar had been achieved before the end example, while Madagascar’s endemic terres- of the Eocene (Schwartz & Tattersall 1985), trial mammals consist today only of strepsir- and probably well before. A l t e r n a t i v e l y, the hine primates, nesomyine rodents, tenrecid close relationship between the Malagasy insectivores, and viverrid carnivores, each of cheirogaleid lemurs and the African lorisoid these groups is present in impressive variety. primates might imply an initial colonization No scenario aimed at explaining this remark- of Madagascar by ancestral African strepsir- able faunal makeup can ignore the question hines, followed by the evolution in of Madagascar’s geological origin; and al- Madagascar of the ancestral lorisoid with a though the details of this origin remain an subsequent back-migration to A f r i c a . arena for debate, it is by now generally agreed that from a middle Jurassic location Under conventional paleogeographic recon- adjacent to the modern Ta n z a n i a / K e n y a / structions such invasions must have been via Somali coast well to the north of its present a ‘sweepstakes’ mechanism; and among emplacement (Reeves et al. 1 9 8 7 ) , mechanisms of this kind rafting is the only Madagascar had assumed more or less its remotely plausible alternative, despite various current position relative to Africa at some factors that point to its inherent improbabili- time prior to the late Cretaceous (Cochran t y, particularly in an eastward direction. 1988), over 120 My. Direct continuity Lawlor (1986), for instance, has demonstra- between the African and Malagasy biotas was ted the extreme rarity with which terrestrial thus ruptured by around 160 My, although mammals have successfully colonized isola- certain paleogeographic reconstructions ted islands anywhere. In a situation of this suggest that indirect interchange via South kind one can only follow Sherlock Holmes America, Antarctica and India might have and conclude that, once the impossible has been available until about 130 My (see dis- been eliminated, what remains, however imp- cussion by Krause et al. 1997), or even much lausible, must be what happened. Yet if raf- more recently, in the 100-80 My range ting of primates across a Mozambique (Sampson et al. 1 9 9 8 ) . Channel of roughly modern dimensions was possible during the Eocene or earlier, why These observations effectively eliminate the not subsequently? After all, among various older notion that Madagascar’s mammal other forms, the progenitors of the endemic fauna, depauperate in major taxa but rich in viverrids of Madagascar must have entered endemic species, is a direct insular relic of the island by the same means subsequent to A f r i c a ’s early Tertiary biota. For while there the Oligocene, when the oceanic barrier was can be little doubt that Madagascar’s mam- apparently no more formidable than earlier in mals are of African parentage, the island was the Te r t i a r y. As long as we adhere to a scena- clearly separated from the mainland by a sub- rio of unchanging biogeography, satisfactory stantial oceanic barrier by the early Te r t i a r y potential explanations for the cessation of pri- date at which we can reasonably suppose that mate penetration of Madagascar subsequent the parent stocks of the surviving Malagasy to some point early in the Tertiary are few, mammal families had evolved in Africa. T h e and none is compelling. Perhaps, for reasons almost complete absence of pre-late Eocene of chance in a situation already close to the mammal fossil records in Africa, Madagascar outer limits of probability, no later crossings and India makes precise paleobiogeographi- were made. A l t e r n a t i v e l y, in contradiction to 304 TATTERSALL: Madagascar the hallowed notion that strepsirhines were Madagascar by the ancestors of today’s ende- able to diversify in Madagascar only due to mic Malagasy terrestrial mammals, and at its the absence of competition from competitive- limit it would even introduce the possibility ly superior ‘higher’ primates, is it possible of a more or less continuous land bridge from that Madagascar’s strepsirhines were actually the continent to the island at least periodical- successful in outcompeting later arrivals? ly in the 45-26 Ma time window. However, it should be noted that the existence of a land The reality may, however, have been totally bridge of this kind would raise as many pro- d i ffe rent. For recently the possibility has blems as it solves. For, almost regardless of been raised again that paleogeography across the ecological conditions that reigned along the Mozambique Channel between it, a continuous bridge would be expected to Madagascar and Africa has not in fact remai- have had a substantially less dramatic filte- ned stable throughout the Te r t i a r y. T h e ring effect than Madagascar’s modern fauna Channel is today a 400-600 km-wide swath implies. Such a bridge, if it existed, was cer- of water of oceanic depth, with a handful of tainly of an appropriate age to explain the small and relatively recent volcanic islands presence in Madagascar of the island’s ende- and atolls.